ORIGIN OF BLOOD AND ENDOTHELIUM 269 



These appearances, as stated above, are not readily distin- 

 guishable in the very young embryos, yet with a little experience 

 and a high power microscope any one may convince himself 

 that the blood island formation proceeds to a very definite 

 and considerable degree in these embryos. 



Figure 34 represents a cross-section through the yolk-sac of 

 an embryo of seventy-two hours old. The ectoderm of the 

 yolk-sac now becomes two-layered, this continues to thicken 

 as age advances until finally in old embryos the yolk-sac ectoderm 

 is many cells thick and often folded and complex in arrangement 

 sometimes showing villus-like processes. Beneath the ectoderm 

 a group of early erythroblasts or blood cells is illustrated. These 

 cells lie immediately upon the yolk mass here indicated by the 

 heavy dark granules. The appearance of the cells in this blood 

 island anlage are closely similar to those shown in cross sections 

 of the intermediate cell mass in figures 31 and 32. The cell 

 nuclei and general cellular arrangements of the two tissues are 

 seen to correspond in appearance, and the manner of differentia- 

 tion followed in both cases is identical. 



In figure 35, a group of five early erythroblasts are shown which 

 were present in a neighboring blood island. They had loosened 

 themselves from the general island mass and appear very much, 

 if not exactly, similar to the early erythroblast seen separating 

 themselves from the compact mass, the intermediate cell mass 

 (fig. 32). The nuclei in all cases are typically those of early red 

 blood cells and the cytoplasm just begins to stain a very pale 

 pink color characteristic of the halo seen around the young 

 erythroblast. All of the cells shown in these yolk islands, both in 

 the earliest condition of the island and in the late old yolk 

 vessels of embryos without a circulation are invariably of the 

 erythroblast or erythrocyte type. In no case has any type of 

 lymphocyte or leucocyte been present in these yolk islands ex- 

 cept as late wandering cells. 



Not all of the wandering cells which are found on the yolk- 

 sac go to form blood cells since many of them are future chroma- 

 tophores or future endothelial vessel cells. The types, how- 

 ever, are distinguishable in rather early stages and do not seem 



